This invention relates generally to medical apparatus for use in extracting liquids from, or injecting liquids into, a living body and, more particularly, to medical apparatus of this kind that are filled with a storage liquid for extended time periods.
Medical apparatus of this particular kind can take many forms, including that of a blood chemistry sensor assembly of the kind disclosed in U.S. Pat. No. 5,165,406, issued in the name of David K. Wong and entitled "Electrochemical Sensor Apparatus and Method." The disclosed blood chemistry sensor assembly includes a rigid plastic housing that defines an elongate test chamber through which passes either blood or a calibration fluid. A plurality of electrochemical sensors are located within the housing, adjacent to the test chamber, each sensor being adapted to measure a separate parameter of the adjacent fluid, e.g., oxygen partial pressure (pO.sub.2), carbon dioxide partial pressure (pCO.sub.2), glucose, hematocrit, calcium, hydrogen ion (i.e., pH), chloride, potassium, and sodium.
Prior to its being used to measure various parameters of a blood sample, the sensor assembly described briefly above typically is stored for an extended time period, with a storage liquid being located within the assembly's test chamber and with the entire assembly being stored within a sealed container of low permeability. The presence of the storage liquid prevents the sensors from drying out, which could adversely affect their sensing capability, even if subsequently re-wetted. Over time, however, the liquid can diffuse from the test chamber, to at least a limited extent, through the assembly's plastic housing and associated tubing, etc.
The diffusion of liquid from the test chamber can give rise to two problems. First, the resulting negative pressure can draw air or other gas into the test chamber, to form bubbles in the remaining liquid. These gas bubbles can contribute to drying of the sensors, which adversely affects the sensors' performance, even if the bubbles are flushed out prior to use. Further, the diffusion of liquid from the test chamber can change the composition of the remaining liquid. In many circumstances, this change can adversely affect the accuracy of the subsequent sensor measurements.
Other kinds of medical apparatus incorporating liquid-filled chambers that must be stored for extended time periods include ready-mix medication containers configured to directly administer the medication to a patient intravenously, and reagent-containing diagnostic kits for various analytes, etc.
It should, therefore, be appreciated that there is a need for medical apparatus of the kind described above, wherein the apparatus can be reliably stored over an extended time period, without risk of air or other gas entering the apparatus' chamber., and without substantial change to the composition of the storage liquid. The present invention fulfills this need and provides further related advantages.